I am experiencing slowness with Spectralayers 12 too, just upgraded from v11.
An mp3 that is 3:52 long as an example, on a Win 10 (latest), AMD 5950X, 64GB RAM, configured to use NVidia 4070 for unmixing at High in v12 or Extreme in v11 settings:
Spectralayers 11: 1:46
Spectralayers 12: 3:36
What’s more, the quality in v11 is better! Gone backwards!!